The present invention relates to metal plating and more particularly to the co-deposition of fluorinated carbon and a diamond-containing material with electroless metal platings.
The field of electroless plating of metals is now well established, having begun in the 1940's at the United States National Bureau of Standard. Electroless metal plating is now widely used to deposit nickel, copper and gold platings in a variety of applications. In addition to nickel, copper and gold, it is also possible to deposit metals including palladium, cobalt, silver and tin, although the use of the latter metals is not nearly as widespread. The most widely used electroless metal deposition is nickel.
As is now well known and understood in the art, electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions plated on a base substrate. Deposits made by electroless plating have unique metallurgical characteristics. The coating formed thereby has uniformity, excellent corrosion resistance, wear and. abrasion resistance, nonmagnetic and magnetic properties, solderability, high hardness, excellent adhesion, low coefficient of friction and like properties as are understood in the art. Such deposits can be made onto a wide range of substrates, both metallic and nonmetallic.
Electroless bath compositions have likewise been well established in the prior art. Such baths typically contain an aqueous solution of metal ions to be deposited, catalysts, one or more reducing agents, one or more complexing agents and bath stabilizers, all of which are tailored to specific metal ion concentration, temperature and pH range. In electroless metal depositing, use is made of a chemical reducing agent, thus avoiding the need to employ a electrical current as required in conventional electroplating. Because the deposit is made from a bath, the deposit follows the contours of the substrate, without build-up at edges or corners of the substrate. A sharp edge receives the same thickness of deposit as a blind hole. Because the deposit is autocatalytic, the base substrate itself is preferably catalytic in nature, causing the reaction to occur once the base substrate is immersed in the bath to form a uniform deposit on the surfaces thereof.
In the electroless plating process, metal ions are reduced to metal through the action of chemical reducing agents serving as electron donors. The metal ions are electronic acceptors which react with the electron donors to form a metal which becomes deposited on the substrate. The catalyst is simply the substance, the workpiece or metallic surface provided to the bath, which serves to accelerate the electroless chemical reaction to allow oxidation and reduction of the metal ion to metal.
The following chemical formulae illustrate an "electroless reaction", i.e., electroless nickel (sodium hyphophosphite reduced) acid bath: ##STR1##
The metal ion and reduced concentration must be monitored and controlled closely in order to maintain proper ratios and to maintain the overall chemical balance of the plating bath. The electroless plating deposition rate is controlled by temperature, pH and metal ion/reducing agent concentration. Each of the particular plating reactions has optimum ranges at which the bath should be operated.
Complexing agent(s) act as a buffer to help control pH and maintain control over the "free" metal salt ions available to the solution, thus allowing solution stability. The stabilizer(s) act as catalytic inhibitors, retarding potential spontaneous decomposition of the electroless bath.
Few stabilizers are used in excess of 10 PPM, because an electroless bath has a maximum tolerance to a given stabilizer. Excessive use of stabilization materials can result in depletion of plating rate, bath life and poor metallurgical deposit properties.
Trace impurities and organic contamination (i.e., degreasing solvents, oil residues, mold releases) in the plating bath will affect deposit properties and appearance. Foreign inorganic ions (i.e., heavy metals) can have an equal effect. Improper balance and control will cause deposit roughness, porosity, changes in final color, foreign inclusions and poor adhesion.
It is also known that various materials can be co-deposited in the formation of electroless metal coatings. U.S. Pat. No. 3,753,667 discloses a process for the electroless coating in which the nonmetallic, wear-resistant material is co-deposited with, for example, nickel in an electroless system. The wear resisting particles described are inorganic particles such as kaolin, silicates, as well as fluorides of various metals such as aluminum, boron, chromium and like metals. Similar teachings are contained in U.S. Pat. Nos. 4,997,686 and 5,145,517. The latter patents refer to co-depositing "particulate matter" with electroless deposition for the purpose of providing lubricity and resistance to wear, abrasion and corrosion. The latter patents include, as an essential component, a complex mixture of what the patents refer to as "particulate matter stabilizers" for the purpose of causing a significant shift in the zeta potential. Those stabilizers are surfactants, and the patents require a mixture of a nonionic surfactant in combination with another surfactant selected from the group consisting of anionic, cationic and amphoteric surfactants.
Substantial improvements over the subject matter of the latter two patents are disclosed in U.S. Pat. No. 4,830,889. That patent describes an improved process for depositing fluorinated carbon in an electroless metal plating process necessitating a combination of surfactants which include a non-ionic, non-fluorinated surfactant in combination with a cationic fluorinated surfactant, and preferably a cationic fluorinated in the form of an alkyl quaternary ammonium iodide surfactant.
It is accordingly an object of the present invention to provide a process for use in the electroless plating of metals which overcomes the foregoing disadvantages.
It is yet another object of the invention to provide a process for the co-deposition of two or more particulate matters insoluble in the bath for use in the electroless plating of various metals.
It is a more specific object of the present invention to provide a process for the electroless deposition of metals which are co-deposited with both fluorinated carbon and diamond-containing material wherein the diamond particles have a minimum average size to aid in the stability of the bath and the codeposition of the diamond-containing material with the fluorinated carbon.
These and other objects and advantages of the present invention will appear more fully hereinafter from the following description which is provided by way of illustration and not by way of limitation of the practice of the present invention.